In an optical system of a light scanning unit using a laser as a light source, a laser beam emitted from the laser is collimated by a collimator lens, and a to-be-scanned surface is scanned with the collimated laser beam through a rotary polygon mirror and an Fθ lens, while the laser beam is imaged on the to-be-scanned surface through a cylindrical lens and the Fθ lens.
Recently in a light scanning unit for use in an image forming apparatus or the like, there has been known a fundamental configuration in which a plurality of arrayed light sources are used for increasing the printing speed, and a coupling optics, a cylindrical lens and a spherical lens are disposed between the light sources and an optical deflecting element so that a pupil plane on a posterior focal plane of an imaging optics and the optical deflecting element have a conjugate relationship (for example, see JP-A-63-208021).
Further, due to growing requirements for higher definition in printing quality, enlargement in printing width, higher speed in printing speed, and so on, the imaging performance of optical systems has been severer and severer. The imaging performance of an optical system depends on the machining and assembling accuracies of lenses, mirrors, etc., the aberrations of the lenses and the mirrors, and the diffractions of laser beams. Therefore, not only improvement in machining accuracies of the lenses, the mirrors, etc., but also improvement in the assembling and adjusting accuracies of those are required.
One of factors in causing deterioration of the imaging performance of an optical system is a problem that the diameters of light beams on a to-be-scanned surface are increased because so-called torsion which is a two-dimensional asymmetry occurs in the light beams due to rotation of a cylindrical lens. A solution to this has been disclosed, i.e. by adjustment of the rotation angle of the cylindrical lens with its optical axis as its rotation axis, the quantity of the torsion can be suppressed (for example, see JP-A-7-248461 and JP-A-6-18801).
However, when the rotation angle of the cylindrical lens is adjusted as described in JP-A-7-248461 and JP-A-6-18801, not only is there a problem that a device for adjusting the angle is required, but there is another problem that it takes time to adjust the angle or the number of man-hours in production increases.